Spare parts inventory and cycle time reduction

Today many manufacturing firms are focusing on cycle or throughput time reduction to gain a competitive advantage. In many instances, cycle times at individual workstations are adversely affected by unscheduled breakdowns. In this paper, existing closed-form approximations for the expected cycle time at a parallel machine workstation with general process and interarrival times and for the variability in departures from the workstation are used to demonstrate how the mean and variance of machine repair of downtime affects cycle times at a given workstation and downstream workstations. Results from the machine interference literature are used to characterize the dependence of repair time on a base stock inventory policy used for the replenishment of spare components. Expressions for the mean and approximations for the variance of repair times are obtained for machines with either a single critical component or a series of critical components when such components fail exponentially. Numerical examples are given to illustrate the effect that spare component inventory levels have on the average station cycle time and departure variability. A methodology for base stock parameter selection that considers the trade-off between reduced cycle times and inventory investment is also presented.